Topological codes [1]}, [2]}, [3]}, [4]}, [5]}, a specific class of scalable QECCs, are currently being studied as one of the main options for future QEC as they have high error thresholds. Moreover, they are designed such that the physical qubits only need to be manipulated locally, i.e. requiring interactions between neighbours. Local errors that stem from such manipulations can then be efficiently detected and corrected by the QECC. To achieve this scalable protection, the lattices where the physical qubits reside must be enlarged, and the fidelity of the operations on the physical qubits needs to be reduced below the aforementioned threshold. Some experiments have already demonstrated levels of control on the required regime for small systems (e.g.  [6]}), but scaling up the experiments while maintaining the desired degree of precision remains a challenge. To tackle this, various low-distance QEC codes at the reach of different state-of-the-art technologies, are being actively investigated. For such low-distance codes, it is of primary importance to use fault-tolerant (FT) circuit designs so that errors do not proliferate due to unsuitable circuit layouts, allowing to exploit the full correcting power of the QECC. Such fault-tolerant circuit constructions will serve the purpose of reaching the break-even point of the advantage of QEC in small devices and provide crucial information that can guide future strategies in progress towards large-scale quantum computers. The 7-qubit colour code [3]}, also known as the Steane code  [8]}, is one of the workhorses for such low-distance QECCs, and various techniques for an FT syndrome readout have been devised [9]}, [10]}. Due to present experimental capabilities, there has been intense recent activity developing resource-efficient FT schemes, such as FT readout techniques that exploit a single ancilla for certain types of microscopic noise [11]}, [12]}, [13]}, [14]}. Another example is using the so-called flag-based readout [15]}, [4]}, [17]}, [18]}, which substantially reduces the number of required ancillary qubits, encompassing noise models beyond the single-qubit limit. These FT constructions exploit the full correcting power of the 7-qubit colour code, which allows for the correction of any single fault in the encoded logical qubit. In turn, this yields a different scaling of the logical error rate to the physical error rate, which allows for the existence of pseudo-thresholds and is crucial for the demonstration of the advantage of QEC with small-scale devices. Colour codes distinguish themselves from other existing QECCs because of their capability of a transversal implementation of the entire group of Clifford gates [3]}.
